I am an addict of www.dictionary.com, and I freely admit it. You all may not know this, but my ability to ruminate and pontificate in an *almost* intelligent manner are almost 99% in thanks to the thesaurus provided on this site. (The other 1% comes from eating lots of fish.)
Yes, there are books that do this, but really who wants to get up, walk to a bookcase, search out the book, and then page through the damn thing? Not to mention, the book won't suggest other words when you had the spelling wrong!
Thank you, dear internet, for helping me sound smart and stuff. Sometimes. Oh, okay, once and a while. On occasion.